Our objectives are to understand the cellular and molecular mechanisms of protein uptake and protein turnover in higher organisms and the factors which regulate this process. We propose to identify intermediates in the degradation of yeast invertase, a model glycoprotein, subsequent to its uptake by rat liver. In other experiments we employ rat liver cells and rat yolk sacs to study the basis of selectivity in and the factors which regulate pinocytosis. This system will also be employed to study the fate of proteins subsequent to their translocation to the lysosomal compartment.